In coal mining, dust is produced at the face, along conveyors, at loading points, at belt transfer points, in drilling operations, and by the normal movement of men and machines. Although the coarse dust particles settle rapidly, the fine particles remain airborne and are often transported by the ventilating air for relatively long distances into the returns before settling. During mining, coal dust is carried in the air stream and deposited almost continuously, whereas rock dust is applied and deposited intermittently. This results in stratified layers of dust on the ribs, roof and floor, rather than homogeneous mixtures of coal and rock dust. These deposited particles include a reactive fraction called "float coal dust" which consist of particles of coal that when dry pass through a no. 200 sieve (74 micrometers).
The degree of explosion hazard posed by this reactive dust is related to the concentration of float coal lying on top of a properly rock dusted substratum. Ignition of a relatively small amount of methane-air or dust-air mixture can start a small explosion that entrails coal dust from the mine surfaces, disperses it into the air and ignites it. In the U.S. the coal dust deposits are usually protected by generalized rock dusting. The incombustible amount of dust is required by law to be 80 percent in a return airway. In the presence of methane, the amount of inert rock dust needed is increased by 1 percent for each 0.4% methane in the ventilating air. However, if a fresh layer of coal dust greater than about 50 mg/l is deposited on top of a properly (80% rock dust) rock dusted substratum, this new layer can be skimmed off by a relatively weak methane-air explosion. Such a hazardous coal dust layer is only 0.124 millimeter thick.
Mine operators, in order to comply with the law and provide protection against such an occurrence, place a trickle rock duster in the returns to periodically liberate rock dust into the ventilating air. Other methods include periodic raking of the entry or generalized re-rock dusting of the entire entry. Presently, very little actual data are available regarding float dust deposition rates in U.S. mines and operators are forced to rely on visual observations to determine the severity of the float-dust deposits. Therefore, the frequency with which new rock dust should be dispersed or when raking should be done to protect the new coal dust deposit is unknown. The result is that the current rock dusting practices cause some mine operators to rock dust excessively in some areas at the expense of others.
There are disclosed in the prior art several devices for detecting particle concentrations or thickness of particle layers. For example, U.S. Pat. No. 4,420,256 (Fladda et al.) discloses a measuring apparatus for obtaining concentration and/or particle diameter of dust particles in a flowing medium. This apparatus obtains these measurements by illuminating the flowing particles with light in a direction different from the direction of flow.
In U.S. Pat. No. 3,647,301 (Sturzinger), a system is disclosed for determining a layer thickness in the accumulation of pulverulent, granular or flaky goods. The system monitors reflected light from the good deposited on a moving surface and uses an optical system to measure the layer thickness of the accumulation.
In U.S. Pat. No. 4,474,472 (Winter), a measuring system is disclosed for detecting particles in a gas flow, such as toner particles which might be found on a photoconductor drum in a non-mechanical fast printer. This system provides a light source together with a light detector so that the detected light as influenced by the particles controls a connected evaluating circuit.
In U.S. Pat. No. 3,564,263 (Shaw) and 3,810,617 (Steinberg), apparatuses for determining the concentration of particles in a fluid stream are disclosed. The apparatuses use light rays which are directed through the stream.
A method for determining the inert content in coal dust/inert dust mixtures has been disclosed in a co-pending application (U.S. Ser. No. 943,347, filed Dec. 19, 1986 which measures the concentration of inert dust in a homogenous mixture of coal dust and inert rock dust. That method thus involves taking samples of coal dust and rock dust and combining the two into a mixture before a determination of concentration is made and does not measure surface densities of layers of dust while settled in various places in an underground mine.
The prior art thus does not disclose a method by which the surface density of stratified layers of explosive float coal dust and the inert rock dust deposited at different locations in an underground mine can be measured quickly, safely and accurately.